1. Field of the Invention
This invention relates to a readily adhesive polyester film and a process for preparation thereof, and relates more detailedly to a polyester film having an excellent adhesive property and excellent blocking resistance, useful as a base material of highly processed film articles such as, for example, magnetic tapes for audio, magnetic tapes for video, magnetic tapes for computer, floppy disks, X-ray photograph films, telephone cards, membranes and diazomicrofilms, and particularly magnetic recording media; and a process for preparation thereof.
2. Description of the Prior Art
It is known to melting extrude and then subject to film formation a thermoplastic polyester, for example, polyethylene terephthalate or its copolymer, polyethylene naphthalate or its copolymer, or a blend of one of them and another resin in a small rate, or the like. It is also known that the resulting biaxially stretched and thermofixed polyester film is superior to a film consisting of another resin in heat resistance, gas screening ability, electric characteristic and chemical resistance. However, since its surface is in the state of high crystalline orientation, it has a high cohesiveness and is thus poor in an adhesive property to paints, adhesives, inks, etc.
As methods for improvement of this adhesive property, there are known physical treatments such as, for example, corona treatment, treatment with ultraviolet ray, plasma treatment, Electron Beam treatment and flame treatment; and chemical treatments, for example, treatments with chemicals such as alkali, an aqueous amine solution, trichloroacetic acid and phenols. However, these methods have practically disadvantageous problems such as degradation of the adhesive force with time lapse and contamination of the working environment by volatilization of the chemicals.
As an alternative means, a method is known which comprises applying a readily adhesive paint onto the surface of polyester film in a process other than the usual film-forming step of the polyester film to provide thereon a primer layer. However, in this method, an organic solvent is usually used as the solvent in the paint and moreover it is hard to say that the coating atmosphere is sufficiently clean, and thus there are problems on safety and hygiene such as frequent occurrence of surface defects of processed commodities due to adhesion of dust and aggravation of the environment due to the organic solvents.
Incidentally, in case where this primer treatment is carried out during the film-forming steps of the polyester using an aqueous paint, there occurs no adhesion of dust in the clean environment and because of the aqueous solvent there is no fear of explosion and aggravation of the environment, and thus it is advantageous on the points of performance.
From such advantages, it has been proposed to use as a primer water soluble or water dispersible polyester reins or acrylic resins (Japanese Laid-Open Patent Publication No. 43017/1979, Japanese Patent Publication No. 10243/1974, Japanese Laid-Open Patent Publication Nos. 19786/1977 and 19787/1977, etc.). However, the above polyester resins have a disadvantage that when the resulting film is wound into rolls, it easily causes blocking, and the above acrylic resins have a disadvantage that they are sufficient in the adhesive property with the base film, mechanical strength and the adhesive property with the magnetic layer. For the purpose of improving these disadvantages, it has been proposed to use the above polyester resin and the above acrylic resin in mixing thereof (Japanese Laid-Open Patent Publication No. 124651/19.83), but it is hard to say that these disadvantages are sufficiently improved.